Journal of Metals, Materials and Minerals, Vol. 23 No. 2 pp. 37-41, 2013 Crystallization of carnallite from KCl-MgCl2 brine solutions by solvent evaporation process and its structural and mechanical characterization Jiban PODDER 1, Shaoyu Gao 2, Richard W. Evitts 1*, Robert W. Besant 2, and D. Matthews 3 1 Department of Chemical and Biological Engineering, 2 Department of Mechanical Engineering, 3 PCS Pilot Plant, Innovation place, Saskatoon, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada Abstract Carnallite crystals have been synthesized from brine solutions at room temperature (23°C) by solvent evaporation growth technique to show the effect of the concentration of potassium chloride (KCl) and magnesium chloride (MgCl2) on the growth, structural and mechanical properties of the crystal. Transparent and pseudo-hexagonal shape carnallite crystals with the dimension of 4 5 2 mm to 16 13 3 mm were grown. The crystals were characterized by powder X-ray diffraction and the lattice parameters were found to be a = 9.598 Å, b = 16.141Å and c = 22.519Å, space group: Pnna, Z: 12, α = β = γ = 90° with the unit cell volume, V = 3488.67Å3. The lattice parameters are found to be in good agreement with the standard values and the structure of the crystal is orthorhombic. A Vickers micro hardness test was carried out to determine the hardness number and work hardening coefficient of the carnallite crystals. Keywords: Carnallite crystal; Potassium-magnesium chloride solution; Solvent evaporation; X-ray diffraction, Orthorhombic structu *Introduction Carnallite crystals are transparent to translucent, optically biaxial (positive) and anisotropic crystals. The study of multicomponent brine solutions is Its crystal habits are typically granular and massive, an important area of research from extraction point and sometimes fibrous. Individual carnallite crystals of view by flotation or recrystallization process in are pseudo- hexagonal orthorhombic and tabular but the mineral industries. Mined and processed potash are extremely rare (3). is usually 98% pure KCl and widely used as a source Currently there is lack of data on the crystallization of potassium fertilizers. Potash ore is composed of of carnallite in a laboratory. A very small carnallite approximately 55% (w/w) NaCl, 40% (w/w) KCl crystal of 1-2 mm size has grown after a few days in a and remaining 5% (w/w) is non-soluble clay and laboratory (4). Carnallite crystal was synthesized from a non-clay materials as impurities (1). The most solution by slow crystallization at 25C and determined important mineral salts from solution mining are the lattice parameters and the space group of this halite, sylvinite and carnallite. Carnallite is an crystal (5). The new process of synthetic carnallite important source of potash, an invaluable fertilizer production by spent magnesium electrolyte (SME) also. Carnallite occupies the void spaces between method was reported and the optimum SME particle halite crystals and exists in the form of microcrystals size was obtained 0.315 mm (6). So far our knowledge (2). mostly on the surface of the processed KCl is concerned; no report is available on the growth of In nature, carnallite is formed with a perfect pseudo-hexagonal structure of carnallite crystals with a orthorhombic structure and is sometimes blue, yellow large size under solvent evaporation technique at room and pink in colour. Carnallite is a hydrated double temperature. However, from a processing point of chloride mineral salts (potassium-magnesium chloride) view, it is of scientific interest to investigate the with six hydrated molecule water (KMgCl3·6H2O) carnallite crystal and to find mechanisms to recover and it only forms in nature under a specific environmental KCl from carnallite based on the concentration gradient condition in an evaporating sea or sedimentary basin. of the brine solutions. In addition to technological and application point of interest, Carnallite crystals have potential mineral sources. In order to understand the * Corresponding author Email: [email protected] J. Met. Mater.Miner. 23(2). 2013 38 PODDER, J., et al. mechanisms of carnallite formation, we have attempted with an operating voltage of 40kV and current of 45 mA. to grow the carnallite crystals in a laboratory from Dried samples for XRF analysis are pulverized in a aqueous solutions by solvent evaporation process at Retsch ZM-200 rotor mill, the pulverized sample is room temperature and investigate the structural and then pressed at 30,000 pounds pressure in an Angstrom mechanical properties of the crystals. press to form a 40 mm diameter pellet. XRF analysis of the pressed pellets is performed using a PANalytcal Materials and Experimantal Procedures PW2424 Magix wavelength dispersive XRF. Analar grades KCl, MgCl2.6H2O supplied by Results and Discussion Fisher Scientific were used and a saturated solution was prepared at 23C (room temperature) containing Crystallization of Carnallite 2.5 mole KCl and 90 mole MgCl2.6H2O following the Van’t Hoff phase diagram (7). The prepared solution Good quality transparent defect free seed was filtered and kept in a petridish covered with a crystals were grown in a Petri dish by a solvent perforated polythene sheet to permit evaporation of evaporation technique at room temperature. solvent in a dust free environment. Care was taken to Optically good crystals having the dimensions of 4 provide an atmospheric ambient devoid of gusts of air 5 2 mm to 16 13 3 mm with perfect external current. Seed crystals were prepared at room temperature morphology were harvested within a period of 3 to 4 by spontaneous nucleation. After preparing the seed weeks and are shown in Figure 1(a), 1(b) and 1(c). crystals, mixed solutions were prepared with different Table 1 gives the chemical composition of the compositions of KCl and MgCl2.6H2O Seed crystals synthesized carnallite and the product yield from the with perfect shape and free from macro defects were solution mixture. To warm up the solutions and for a used for the experiments. little bit faster evaporation, a 60 Watt lamp was used and focused to the solutions taken in a Petri dish Characterizations covered by perforated polythene sheet (as carnallite is an evaporate mineral) The amount of chlorine, magnesium, potassium In our experiment we observed that due to the was determined using X-ray fluorescence spectroscopy different solubilities of magnesium chloride and (XRF). Furthermore, to determine the lattice parameters, potassium chloride in water solvent, the mixed a powder XRD analysis were performed using an solution began to precipitate out as crystals once automated ‘X’-Pert PRO MPD Theta-Theta system saturation has been reached. Carnallite starts to (PANalytical) X-ray powder diffractometer with ‘X’ precipitate from mixed solution when temperature is Celerator RTMS (Real Time Multiple Strip) detector reduced. However, such mixed solutions must be and monochromatic CoKα radiation (λ=1.70688 Å) close to saturation before precipitation. Figure 1. Photographs of (a) carnallite seed crystals, (b) & (c) pseudo-hexagonal carnallite crystals, (d) Seed crystals of magnesium chloride hexahydrate, (e) magnesium chloride hexahydrate crystals. J. Met. Mater.Miner. 23(2). 2013 Crystallization of carnallite from KCl-MgCl2 brine solutions by solvent evaporation process 39 and its structural and mechanical characterization Table 1. Chemical composition of synthesized carnallite (wt%) by XRD analysis. Sample X-ray florescence analysis (%) X-ray diffraction study (%) wt% /50 ml water K Mg Cl H2O Product 1.12 (g):110.38 (g) MgCl2.6H2O 14.70 7.60 39.50 35.47 Carnallite (97.27 %) 1.10 (g) KCl:160.26 (g) MgCl2.6H2O 0.00 11.72 34.66 53.62 MgCl2.6H2O (99.40%) 10.8 (g) KCl: 29.2 (g) MgCl2.6H2O 15.20 7.23 37.65 39.92 Mixed KCl & MgCl2.6H2O Pure carnallite 13.51 8.80 38.16 39.51 100 At the surface of the mixed solution (i.e. air-liquid solution have reversed the charge on the KCl, thus interface), nucleation of small carnallite crystals will reducing the growth of KCl from the mixed appear after attaining super saturation by the solvent solutions. On the other hand a higher concentration evaporation process. Hence they grow at the edges of MgCl2 in solution would reduce the dissolution by additions from the surrounding supersaturated rate of carnallite and with the addition of more water; solution, and formed thin square tablets. These tablets carnallite produces a lower level of supersaturation. typically have depressed centers, as a result of reduced Under such crystallization conditions, the growth of buoyancy as they grow heavier and eventually grow KCl will be reduced. Thus there are three possible ways too heavy for the surface tension forces; ultimately to precipitate carnallite from brine, including evaporative they drop on the bottom of the Petri dish. The solubility concentration of intercrystalline brine, cooling and of KCl is low in concentrated magnesium chloride mixing of brines of appropriate compositions. solutions, so initially very high magnesium chloride concentrations is not suitable for the conversion of Powder X-ray diffraction carnallite. Therefore, the solubility, temperature and mixing intensity of KCl is an important in magnesium The powder XRD pattern of carnallite crystals chloride solutions for the conversion of carnallite. The are shown in Figure 2. The interplanar spacing (d), concentration of KCl-MgCl2 has to be maintained full width at half maximum (FWHM) and peak stoichimteric otherwise the KCl may be undersaturated intensity (I) of major planes have been identified and in MgCl2 solutions. If it is undersaturated then the are shown in Table 2. The lattice parameters were crystals form almost pure hexahydrate magnesium calculated and presented in Table 3. From the data, it chloride. The lack of produced carnallite crystal could is observed that carnallite crystallizes orthorhombically. be due to its low crystallization rate. Since magnesium The values of lattice parameters for the carnallite 2+ ions exist in aqueous solution normally as [Mg(H2O)6] crystals are found to be in good agreement with the and this promotes the formation of salts having six reported literature (8).